Samples of high-silica SSZ-13, ion exchanged with protons and alkali-metal cations Li+, Na+, and K+, were investigated using adsorption isotherms of CO2 and N-2. The results show that Li-, Na-SSZ-13 have excellent CO2 capacity at ambient temperature and pressure; in general, Li-SSZ-13 shows the highest capacity for N-2., CO2 particularly in the low-pressure region. The effect of cation type and Si/Al ratio (6 and 12) on the adsorption properties was investigated through analysis of adsorption isotherms and heats of adsorption. The separation of CO2 in a flue gas mixture was evaluated for these adsorbents in the pressure swing adsorption and vacuum pressure adsorption processes.